The subject embodiment pertains to the art of printing systems and more particularly printing systems including a plurality of printing engines capable of operating in tandem for parallel or sequential printing of job portions. The preferred embodiments especially relate to a system and method for synchronizing relative operating positions of photoreceptor belts within the printing assembly to avoid undesirable belt seam positioning that can diminish system throughput efficiency. More particularly, this disclosure relates to systems and methods in which image registration tolerances in IOI (Image on Image) color output images are maintained while synchronizing the photoreceptor belts associated with a printing system.
Electrophotography, a method of copying or printing documents, is performed by exposing a light image representation of a desired original image onto a substantially uniformly charged photoreceptor substrate, such as a photoreceptor belt. In response to this light image, the photoreceptor discharges to create an electrostatic latent image of the desired original image on the photoreceptor's surface. Developing material, or toner, is then deposited onto the latent image to form a developed image. The developed image is then transferred to an image receiving substrate. The surface of the photoreceptor is then cleaned to remove residual developing material and the surface as recharged by a charging device in preparation for the production of the next image.
Color images can be produced by repeating the above-described recording process once for each differently-colored toner that is used to make a composite color image. For example, in a one-color imaging process, referred to herein as the Recharge, Expose, and Develop, Image (REaD IOI) process, a charged photoreceptor surface is exposed to a light image that represents a first color. The resulting electrostatic latent image is then developed with a first colored toner. The toner is typically of a subtractive primary color, including magenta, yellow, cyan, or black. The charge, expose and develop process is repeated for a second colored toner, then for a third colored toner, and finally for a fourth colored toner. The four differently-colored toners are placed in superimposed registration on the photoreceptor so that a desired composite color image results. That composite color image is then transferred and fused onto an image receiving substrate.
Printing engines utilizing photoreceptor belts, as opposed to drums, must avoid using the portion of the belt comprising the seam because the seam, if used to store any image data, can mar the output image. In most engine printing systems, paper feeding systems will detect seam position to avoid lining up the paper with the seam. When such avoidance requires delaying the printing operation for the time period of printing a single page, such a wait is referred to as “skipping a pitch” and has a noticeable negative consequence on printing systems throughput efficiency. Adjusting the feed of the paper to assure avoidance of the seam is normally all that is needed in single print engine systems and is usually successful enough so that a pitch is hardly ever skipped.
A special problem exists in multiple print engine systems where a first printing engine (image output terminal or “IOT”) can be a presequential feeder to a second IOT. Of importance is that the second IOT be synchronized to the first IOT, i.e., that the second photoreceptor belt seam is synchronized to the first photoreceptor belt seam to avoid the pitch skipping problems.
When such parallel printing assemblies are initially constructed, it is intended that the respective photoreceptor belts be of the same size (length) and that the motor speed for operating the belts of the IOTs are identical. In such cases, initial calibration is intended to avoid having to adjust the relative positions or operating speeds of the respective engines during operation, or that the feeding system can adjust positions of the documents during input to each engine to accommodate any throughput problems that may arise.
It is an operational objective that there is no delay in paper feed through the system so that throughput can always be maximized. Unfortunately the practical reality is that no two photoreceptor belts are exactly the same size, nor are their respective motors capable of running at exactly the same speed. The respective differences may be quite small, but over time, and the production of many print documents, the respective belts can become so out of synchronization that the conventional paper feed adjustment systems may not be capable of accommodating the phase feed differences and a pitch may have to be skipped.